1. Description of the Invention
This invention relates to a process for producing a coal-water slurry. More particularly it relates to a process for producing a coal-water slurry having a high coal concentration and a good fluidity with a low power consumption for grinding.
2. Related Art Statement
In recent years, in order to improve the handling properties of coal as a solid fuel, technical development has been earnestly carried out in a process for slurrying coal particles by dispersing them in water to thereby convert them into a fluid fuel which can be easily handled. A problem in producing a high concentration, low viscosity and stabilized coal-water slurry which makes it possible to effect direct spray-combustion in the form of a boiler fuel is to devise how to adjust slurry-constituting coal particles so as to have a broad particle size distribution to thereby increase their packing density, resulting in a high concentration thereof and also how to effect stabilized dispersion of coal particles in water by the use of a suitable surfactant and thereby reduce its viscosity.
As for a process for producing such a coal-water slurry, a process of wet-grinding coal by means of a continuous wet ball mill in the presence of a surfactant added to thereby effect stabilized dispersion of coal particles in water has generally been employed.
The above-mentioned mill has usually been composed of a horizontally rotating cylinder and steel balls filled therein for example as disclosed in U.S. Pat. No. 450,041. As to coal grinding by means of such a mill, since the balls lifted along the inner wall of the mill freely drop in the case of a low viscosity of slurry inside the mill i.e. a low coal concentration, the grinding is ruled by impact grinding. On the other hand, in the case of a high viscosity i.e. a high coal concentration, since the motion of the balls inside the mill is restricted to make it impossible to freely drop, the balls flow down while rolling on the surface layer of other balls; hence the grinding is ruled by abrasion. In the case of impact grinding, the resulting coal particles have a narrow particle size distribution, while in the case of abrasion, fine particles are formed and the resulting coal particles have a broad particle size distribution. Thus it is seen that a high coal concentration results in much sion and a broad particle size distribution. However, too high a concentration (usually about 55% or higher) results in a higher viscosity to make proceeding of grinding impossible; hence it is necessary to add a surfactant at the time of grinding.
On the other hand, a dry vertical ring-roll mill has been known as a mill consuming a lower power than dry or wet ball mill. The ring-roll mill results in a broad particle size distribution per unit mill since its grinding mechanism resides intrinsically in compression grinding and frictional grinding. However, since small size particles classified inside the mill are removed at the grinding part and preumatic conveyed in the form of fine powder coal to the outside of the system, these small size particles are not re-ground. Thus, the content of fine particles in the fine powder coal is reduced to give only a narrow particle size distribution. When fine powder coal ground by a ball-race mill as an example of the roll mill was mixed with water and an additive and the mixture was adjusted so as to give a viscosity of 1,500 cP, the resulting slurry concentration was 58% and the specific energy required for the grinding was 15 KWH/t which was 60% of that in the case of grinding by means of dry ball mill. Thus when grinding is carried out by means of ball-race mill, the specific energy required for the grinding can be reduced, but since the slurry properties are far inferior to those exhibited when a high concentration wet ball mill was employed, it is the present status that the resulting slurry is unsuitable as a liquid fuel.
Thus in order to practically use a coal-water slurry as a boiler fuel, a problem is raised that a mill should be developed which can produce a slurry having a broad particle size distribution required for giving a high coal concentration of 60% by weight or higher, with a low power consumption. Further, coal contains a large quantity of ash. Thus in order to be free from environmental pollution at the time of combustion of the coal-water slurry, development of a technique for producing a good quality slurry i.e. a coal-water slurry of low ash content has been desired.
The object of the present invention is to provide a process for producing a coal-water slurry free of the above-mentioned drawbacks i.e. a process for producing a coal-water slurry of low vistosity and high coal concentration with a good efficiency and a low power, consumption.